With the extensive use of personal computers and other data processing facilities both at home and in the office, a need exists for providing voice and data transmission and switching capabilities on a widespread basis. This has led to the development of the concept of an integrated services digital network (ISDN)--a switched communications network providing end-to-end digital connectivity among network users where voice and data services are provided over the same transmission and switching facilities. Because of the different characteristics of voice and data traffic--voice being typically continuous in one direction for relatively long intervals and tolerant of noise but sensitive to variations in delay, and data being bursty and sensitive to errors but tolerant of moderate delays and delay variations--two fundamentally different switching techniques have been traditionally applied. Circuit switching, where switched connections between users are dedicated for call duration, is the basis of the present-day switched voice telecommunication network. On the other hand, packet switching, where data packets from many calls share a single, high-speed line and are switched based on logical channel numbers included in the packets, was pioneered in the ARPANET network of the U.S. Department of Defense, and has now been implemented in a variety of public data networks.
The approach most commonly used at present to serve customers that have borh voice and data requirements is to use separate networks and separate access lines for the two types of traffic. Since both voice and data access lines are typically utilized only a fraction of the time, the duplication of equipment involved in this approach is wasteful and costly compared to an arrangement where facilities could be shared. Some presently available equipment multiplexes both voice and data onto the same access lines. Even if the access lines are shared, however, the voice and data, which are typically transmitted in very different formats, must each be switched to their proper destinations. One approach to the provision of such switching is to use entirely separate packet and circuit switches. This approach, however, is also unnecessarily duplicative. From the line circuits inward, the two information types are served by separate equipment. A primary reason for using this approach is the technical difficulty of handling these two radically different types (and formats) of traffic in an efficient, integrated manner.
Another approach to the problem is to include a centralized, packet switching entity in the switching system and to route all packet traffic from the system users to that centralized entity. However, if circuit switching resources otherwise available for voice calls are used to convey such packet traffic, the switching system voice call capacity is reduced by an amount dependent on the level of packet traffic. Furthermore, the initial cost and complexity of incorporating such a centralized, packet switching entity into the architecture of a circuit switching system is substantial in view of the likely gradual increase in the number of system users requiring packet switching service.
In view of the foregoing, a recognized problem in the art is the difficulty in providing packet switching service to the users of an ISDN switching system in a manner that avoids the inefficiency of unnecessary duplication of equipment and that also minimizes the effect of packet traffic on the circuit-switched call capacity of the system and allows incremental growth in the packet switching capacity of the system as the number of users increases, with minimal impact on the overall system architecture.